Large-amplitude Inertia Gravity Waves over Syowa Station: Comparison of PANSY Radar and ERA5 Reanalysis Data

Abstract

The Graduate University for Advanced Studies, SOKENDAIMaster of Science2024Atmospheric gravity waves (GWs) carry momentum to distant regions and contribute to driving the residual meridional circulation in the middle atmosphere (stratosphere, mesosphere, and lower thermosphere). The residual meridional circulation in the mesosphere forms a characteristic temperature structure with low temperatures at the summer pole and high temperatures at the winter pole owing to adiabatic expansion and compression, respectively. However, GW observations are not enough to verify their behavior especially in the Antarctic, due to the harsh environment there. In addition, GWs have a wide range of horizontal wavelength (i.e., from several km to several thousand km) and period (i.e., from Brunt-Väisälä period (approximately 5 minutes) to inertial period (over 12 hours)), which makes it difficult to reproduce GWs in the entire frequency range even in the state-of-the-art atmospheric models in spite of the recent increase of the model resolution. In order to implement the effect of subgrid scale phenomena into the models, which are not explicitly represented, GW parameterizations are introduced. In general, nonorographic GW parameterization assumes nearly constant wave sources and instantaneous upward propagation, but in reality, the wave sources are not constant and GWs propagate horizontally as well. Thus, it is required to constrain the GW effect in the models based on observations which cover the whole frequency range of GWs and estimate the GW momentum transport in the Antarctic. We examined large-amplitude inertia gravity waves over Syowa Station, Antarctica, using the PANSY (Program of the Antarctic Syowa MST/IS) radar data and the latest reanalysis (ECMWF reanalysis v5; ERA5) from October 2015 to September 2016. Focusing on large-amplitude events with a large absolute momentum flux (AMF), hodograph analysis was applied to both data to estimate the wave parameters. It showed that the inertia GWs with a downward phase velocity becomes dominant in the stratosphere. While their vertical wavelengths got shorter with altitude, their intrinsic periods and horizontal wavelengths got longer with altitude. In addition, their southward propagation was predominant in the stratosphere. While height dependence of the estimated wave parameters is consistent with previous studies investigating inertia GWs over Syowa Station, some features specific to large-amplitude inertia GWs were also observed. The GW features obtained from PANSY were mostly consistent with those from ERA5 except for their amplitudes. On the other hand, comparison of AMF between PANSY and ERA5 indicated that ERA5 significantly underestimated the AMF by a factor of 5 between 5 and 12.5 km altitudes and more above 12.5 km. Difference of horizontal and vertical wind power spectra between PANSY and ERA5 was quantitatively consistent with the difference of AMF and its height dependence. It was found that underestimation of vertical wind spectra primarily contributed to the underestimation of AMF in ERA5. The greater underestimation of AMF in the stratosphere might be due to larger vertical grid spacing in ERA5 and the shorter vertical wavelengths of the dominant GWs in the stratosphere. This thesis is based on Yoshida et al., 2024.thesi